The present invention relates to vehicle wheel alignment, and more particularly to vehicle wheel alignment systems which measure the locations and orientations of the vehicle wheels in a three dimensional coordinate system.
Various systems have been designed to determine vehicle wheel alignment angles. For example, U.S. Pat. No. Re 33,144 to Hunter and Jan. and U.S. Pat. No. 4,319,838 to Grossman and January each describe a wheel alignment system which uses electro-optical transducers to determine the toe alignment angles of a vehicle. FIG. 2 of each of these patents shows six angle transducers carried by support assemblies which are mounted to the vehicle wheels. FIG. 4 of U.S. Pat. No. Re 33,144 and FIG. 9 of U.S. Pat. No. 4,319,838 show the geometry of this arrangement and illustrate the six angles which are directly measured. These patents further describe (see U.S. Pat. No. Re 33,144 col. 7 lines 26-39, and U.S. Pat. No. 4,319,838 col. 8 line 63 to col. 9 line 12) how the toe alignment angles are computed from the angles directly measured by the angle transducers.
U.S. Pat. No. 4,879,670 to Colarelli describes a gravity-referenced inclinometer. FIG. 3 of U.S. Pat. No. 4,879,670 illustrates the mounting of such an inclinometer to a vehicle wheel for measuring the camber of the wheel. The use of gravity-referenced inclinometers to measure camber is conventional, and assumes the vehicle rests while being measured on a surface which is both flat and level.
SAE Publication 850219, titled "Steering Geometry and Caster Measurement", by January, derives and discusses the procedures and methods by which toe and camber alignment transducers are used to determine the caster and steering axis inclination (SAI) of a vehicle. The procedures described therein are the industry standard.
Equipment of this general type and using the apparatus and methods enumerated above has been used world-wide for many years. Such equipment is capable of determining the camber, caster, and pointing or "toe" alignment angles of the wheels relative to one or more appropriate reference axes, and is sufficient to allow proper adjustment of the alignment so as to reduce tire wear and provide for safe handling. It is believed, however, that such equipment could be improved.
U.S. Pat. No. 5,488,472 to January advances the art further by describing the use of conventional toe transducers which, while operating in cooperative pairs, have the additional capability of measuring the distances, each relative to the other. FIG. 7 of U.S. Pat. No. 5,488,472illustrates the use of these "range and bearing" measurements to determine the coordinates and orientations of the sensors and wheels in a two dimensional coordinate system. The disclosure of the '472 patent is hereby incorporated herein by reference. A further description of the prior art may be found therein.
The disclosure of U.S. Pat. No. 5,488,472 illustrates that determining the two dimensional coordinates of the vehicle wheels does not provide greater ability to determine the toe alignments of the wheels relative to the appropriate reference axes of the vehicle. The only determinations of these reference axes which are practical to use are the same as those provided by transducers which measure angles but do not additionally measure distances.
U.S. Pat. Nos. 4,745,469 and 4,899,218, both to Waldecker et al., describe what is commonly known as an "external reference aligner". U.S. Pat. No. 4,899,218 is a continuation of U.S. Pat. No. 4,745,469, and contains no new disclosure. FIGS. 3 through 6 of these patents show how lasers are used to illuminate the tires and cameras are used to examine images of the sidewalls. These patents further describe how "machine vision techniques" are used to examine the images and determine the distances between the cameras and certain locations on the sidewalls, thereby allowing a determination of the locations and orientations of the wheels in a coordinate system which is relative to the cameras.
German Patent DE 29 48 573 A1, assigned to Siemens AG, describes the use of cameras to determine the locations and orientations of the wheels of a vehicle. On each side of the vehicle, a single camera is moved to multiple positions to view the vehicle wheels. Alternatively, a single fixed camera is used at each side in conjunction with movable mirrors, or multiple cameras are used. The system examines the images thus viewed of the wheels of the vehicle to determine the locations and orientations of the wheels, from which the wheel alignment parameters are determined.
European Patent Application PCT/US93/08333, filed in the name of Jackson and published under the Patent Cooperation Treaty as WO 94/05969 (hereinafter referred to as WO document 94/05969), describes the use of a camera having one or more defined fields of view to view optical targets of known configurations which are mounted to the vehicle wheels. Through the use of sophisticated image recognition methods, the three dimensional coordinates and orientations of the vehicle wheels and their corresponding axes of rotation are determined. The wheel alignment parameters are determined from these coordinates and orientations.
There exists a clear need for apparatus and methods which allow a proper determination of the alignment of the vehicle wheels.
In addition to the above-mentioned drawbacks, proper alignment using video systems is critically dependent upon accurate determination of the positions of the targets in the field of view.
It has been discovered that in prior art video alignment systems the brightness of the targets can vary from front to rear of the vehicle. This makes the proper determination of alignment angles even more difficult.
Furthermore, in prior art video alignment systems, the apparent size of the targets varies considerably from front to rear of the vehicle. This can result in reduced resolution for at least one set of wheels.
A further deficiency of prior art video alignment systems arises from the characteristics of lenses used in such systems. A lens cannot be adjusted to have the best possible focus on two separate objects when these objects are separated by some distance in depth unless they are both beyond the hyperfocal distance of the lens. Using a single lens, as in the prior art systems, requires the best focus point to be about 1/3 of the vehicle wheelbase beyond the front target. This means that both the front and rear targets have a slight blur as seen by the camera in prior art systems. In prior art video alignment systems, the resolution of the system is further limited by the need to retain the targets in the field of view of the cameras when the vehicle is raised on a lift rack. For example, at least one prior art system is believed to limit vertical movement of the vehicle to less than eighteen inches. This problem can be alleviated somewhat by making the cameras movable, but that introduces further complications and/or errors.
At least one prior art video alignment system mounts a pair of cameras on a beam disposed at eye height at the front of the vehicle. This results in a hazard for the technician doing the alignment, who must avoid hitting the beam with his or her head. In addition, this system includes lamps adjacent the cameras for illuminating the optical targets. Since the cameras and lamps are disposed so low, the light from the lamps may easily flash in the eyes of the technician with this prior art system.
In some instances, it may be desirable to perform a video alignment measurement of a vehicle in which the vehicle is not raised above ground level, but merely driven through a testing station. Present video alignment systems are not believed to be suitable for such a drive through operation.